1. Field of the Invention
This invention relates to error concealment in digital television signals.
2. Description of the Prior Art
Recently there has been an increasing interest in the use of digital techniques for television signals. Such techniques are, for example, used in some video tape recording arrangements where an incoming television signal to be recorded is sampled, the samples are coded into digital form, the digital data signals are recorded and subsequently reproduced by a video tape recorder (VTR), the reproduced digital data signals are decoded, and the decoded signals are used to form an analog signal corresponding to the original television signal.
If errors occur in the handling of the digital signals, for example due to noise or drop-out occurring in the VTR, the digital signals are lost or corrupted and then the reformed television signal does not correspond exactly to the original television signal, and a resulting television picture is degraded.
There are two main approaches to dealing with errors in digital television signals. The first approach is correction, which involves the production and use of additional data signals purely for the purposes of error detection and correction, these additional data signals otherwise being redundant. While correction provides good results, it cannot generally be used as the sole means of dealing with errors, because a comprehensive correction capability would require an excessive amount of additional data which might overload the data handling paths or raise the data rate to an unacceptable level. The second approach, with which the present invention is more particularly concerned, is concealment. This comprises the replacement of corrupted data signals by data signals generated using available uncorrupted data signals. This method relies largely for accuracy on the strong correlation that exists in a television signal.
In our co-pending UK patent application No. 8011090 (Ser. No. 2073534), the corresponding co-pending European patent application No. 81301156.6 (Ser. No. 0037212), and corresponding U.S. Pat. No. 4,419,693 we have disclosed a method of error concealment which comprises selecting from a plurality of algorithms a preferred algorithm for calculating a corrected value for use in concealment of an error sample, calculating a corrected value for the sample using the preferred algorithm, and replacing the error sample by the corrected value sample. This method works well at the normal reproduction speed, but when reproducing at higher speeds the loss and corruption of reproduced signals as a reproducing head crosses recorded tracks means that not only is correction substantially less effective, but so also is concealment. This is because concealment relies upon the presence of valid samples adjacent to a sample to be concealed, and if a significant number of the adjacent samples have been lost or corrupted, effective concealment is impossible. This problem will be described further below.